The present invention is directed to positive displacement pumps of the general kind disclosed in U.S. Pat. No. 3,168,872 in the name of Pinkerton. As will be more fully described with respect to FIG. 1, Pinkerton includes a closed end cylinder, a piston mounted and driven in a rotary and reciprocating movement in the cylinders. The cylinder is provided with at least a pair of inlet and outlet ports for the admission and expelling of fluid from the cylinder. The piston, which with the cylinder forms a working chamber, includes a flat duct at least at one free end thereof which sequentially communicates with the inlet and outlet ports as the piston is driven through each cycle to form a valveless positive displacement pump.
In numerous types of fluid systems, the intermixing of fluids must be controlled to a high degree of accuracy. One such system for which the present invention is particularly suited is the intermixing of dialysis concentrates with water to yield dialysate solutions, such as in hemodialysis machines.
Hemodialysis machines are utilized by persons having insufficient or inoperative kidney functions. The machines may be used at a health facility or in the patient's home. The machine attaches to the patient through an extracorporeal circuit of blood tubing to a dialyzer having a pair of chambers separated by a thin semi-permeable membrane. The patient's blood is circulated through one of the chambers. The hemodialysis machine maintains a constant flow of a dialysate through the second chamber. Excess water from the blood is removed by ultrafiltration through the membrane and carried out by the dialysate to a drain.
A typical hemodialysis machine provides a pair of hoses which connect to the dialyzer and include a source of incoming water, a heat exchanger and heater for bringing the water to a required temperature, a source of a dialysate concentrate or concentrates which are introduced into the water in a predetermined concentration and necessary pumps, pressure regulators, a deaerator, flow controllers and regulators. In an acetate dialysis system, only one concentrate is utilized, while in the more common bicarbonate dialysis systems, two concentrates, acidified and bicarbonate are utilized.
Accuracy of proportioning of concentrates in such systems commonly is achieved through the use of some type of fixed stroke proportioning pumps, such as diaphragm type pumps. The fixed stroke diaphragm type pumps are operated at varying frequencies to vary the concentrate volumes, but the diaphragm type pumps are not as accurate as piston type pumps. A second commonly utilized piston type pump however, typically is a water driven fixed ratio pump which is not variable, which does not allow for any flexibility of the fluid intermixing ratios. In numerous types of systems it can be important to adjust the amount of one or more fluids independent of one another, such as the concentration of sodium and bicarbonate via volume of the concentrates in the hemodialysis machines.
The positive displacement pump has the capability of providing the precise mixing levels needed, however, the Pinkerton pump has numerous potential problems when utilized in a hemodialysis machine or similar system. The Pinkerton pump, as will be more fully described with respect to FIG. -, can leak, is noisy, does not self align, can jamb due to the buildup of solids and can be inaccurate due to air bubble buildup on the piston duct or due to end stroke changes in volume.